I'll give you ideas i just hope you know there already is a survival island.
One question before i list some ideas: Do you want the map to have a storyline or is it just a survival island?
I'll just make up a quick backstory when I'm done. And there are many survival islands.
I would love to do that, but I'm too laggy to play on servers. Anyways, what can I use to change all land around the island to water? MCEdit?Well then i guess you can just have a limited amount of resources example 2 trees, far away from land mainly sand on the island.
Make the island fairly small, make areas based off the storyline ie. Find the buried treasure.
If you have goals there should be main areas dedicated to those goals.
If you want to make it super hard make it so the water is lava
Make underground mazes etc.
If your going to make any other maps in the future i would really like to help you build them, preferably on a server and not transferring the map back and forth.